Coumarin-based dyes have been successfully used in dye-sensitized solar cells, leading to photovo... more Coumarin-based dyes have been successfully used in dye-sensitized solar cells, leading to photovoltaic conversion efficiencies of up to about 8%. Given the need to better understand the behavior of the dye adsorbed on the TiO 2 nanoparticle, we report results of density functional theory (DFT) and time-dependent DFT (TD-DFT) studies of several coumarin-based dyes, as well as complex systems consisting of the dye bound to a TiO 2 cluster. We provide the electronic structure and simulated UV-Vis spectra of the dyes alone and adsorbed to the cluster and discuss the matching with the solar spectrum. We display the energy level diagrams and the electron density of the key molecular orbitals and analyze the electron transfer from the dye to the oxide. Finally, we compare our theoretical results with the experimental data available and discuss the key issues that influence the device performance.
We report density functional theory (DFT) calculations of three newly synthesized oligomethine cy... more We report density functional theory (DFT) calculations of three newly synthesized oligomethine cyanine-based dyes as potential TiO2-sensitizers in dye-sensitized solar cells. The three dyes have π-symmetry and the same acceptor side, terminating in the carboxylic anchor, but they differ through the π-bridge and the donor groups. We perform DFT and time-dependent DFT studies and present the electronic structure and optical properties of the dyes alone as well as adsorbed to the TiO2 nanocluster, to provide some predictions on the photovoltaic performance of the system. We analyze theoretically the factors that can influence the short circuit current and the open circuit voltage of the dye-sensitized solar cells. We examine the matching of the absorption spectra of the dye and dye-nanocluster system with the solar irradiation spectrum. We display the energy level diagrams and discuss the alignment between the excited state of the dyes and the conduction band edge of the oxide as well ...
Http Dx Doi Org 10 1080 00268976 2013 777811, 2013
h i g h l i g h t s " The silver nanoparticles are obtained using different types of synthetic hu... more h i g h l i g h t s " The silver nanoparticles are obtained using different types of synthetic humic substances. " Synthetic humic substances act as a reducing and stabilizing agents. " The advantage of synthetic humic substances consists in their reproduced properties. " The experimental conditions were optimized to attain better yield, controlled size and high stability. " Obtained silver nanoparticles showed good antimicrobial activity.
We report results of density functional theory (DFT) calculations on some colorless aromatic syst... more We report results of density functional theory (DFT) calculations on some colorless aromatic systems adsorbed on a TiO 2 nanocluster, in order to explain experimental results regarding the photocatalytic degradation of these pollutants under visible light irradiation. Based on our modeling, we are able to clarify why transparent pollutants can degrade under visible light in the presence of a catalyst that absorbs only in the UV, to explain experimental data regarding differences in the efficiency of the degradation process, and to state the key requirements for effective water-cleaning. For that purpose, we analyze the absorption spectrum of the free and adsorbed molecules, the binding configurations, the matching of the energy levels with the oxide catalyst and the likelihood of the charge-transfer to the substrate. The comparison between several colorless aniline and phenolic systems allows a correlation between the chemical structure and the degradation rate of these pollutants.
We report quantum chemical calculations to address yet unresolved and puzzling questions regardin... more We report quantum chemical calculations to address yet unresolved and puzzling questions regarding the structural and magnetic disorder of V[TCNE] x (TCNE = tetracyanoethylene, x $ 2), the first room-temperature molecule-based magnet. Starting from an ideal lattice model, containing TCNE ligands either tetra-or bi-connected to vanadium(II) ions, we identify the key sources of structural disorder, explaining the amorphousness and non-stoichiometric nature of V [TCNE] x . The proposed model is prone to static disorder in terms of the bulk distribution of the tetra-connected TCNE species and to dynamic effects due to the relative rotational freedom of the bi-connected TCNE moieties. Density functional theory (DFT) calculations of the model system with rotated TCNE molecules show a rough energy landscape, consistent with the presence of magnetic irreversibilities in the system. The broken symmetry DFT approach evidences ferrimagnetic spin orientation for all TCNE configurations, ruling out the spin glass model. Multiconfigurational calculations with additional spin-orbit interaction allow for the account of the single-ion-anisotropy of the V(II) ions in different environments. We determine a small uniform zero-field-splitting (D c = À0.03 K) of the bulk as well as a sizeable random anisotropy (D r = 0.56 K) due to TCNE vacancies. We clarify the interplay of ferrimagnetism and random magnetic anisotropy in this system, which favours correlated sperimagnetic and not spin glass behaviour, in agreement with puzzling experimental data. Our approach goes beyond the material of interest here, as it can be applied to other disordered molecular magnets by correlating the sources of disorder with their effects on the magnetic properties.
Coumarin-based dyes have been successfully used in dye-sensitized solar cells, leading to photovo... more Coumarin-based dyes have been successfully used in dye-sensitized solar cells, leading to photovoltaic conversion efficiencies of up to about 8%. Given the need to better understand the behavior of the dye adsorbed on the TiO 2 nanoparticle, we report results of density functional theory (DFT) and time-dependent DFT (TD-DFT) studies of several coumarin-based dyes, as well as complex systems consisting of the dye bound to a TiO 2 cluster. We provide the electronic structure and simulated UV-Vis spectra of the dyes alone and adsorbed to the cluster and discuss the matching with the solar spectrum. We display the energy level diagrams and the electron density of the key molecular orbitals and analyze the electron transfer from the dye to the oxide. Finally, we compare our theoretical results with the experimental data available and discuss the key issues that influence the device performance.
ABSTRACT We report quantum chemical calculations providing the exchange coupling constants of the... more ABSTRACT We report quantum chemical calculations providing the exchange coupling constants of the V[TCNE]2 model system, describing the amorphous room temperature molecular magnet V[TCNE] x (TCNE = tetracyanoethylene, x ~ 2). The geometry is optimized for the ideal lattice using density functional theory (DFT) calculations with periodic boundary conditions. Broken-symmetry DFT calculations indicate antiparallel spin alignment resulting in ferrimagnetic ordering, but heavily overestimate the value of the exchange coupling. Better estimates of the exchange coupling parameters between the V(II) ion and the [TCNE]− anionic radical are obtained by means of multiconfigurational calculations performed on smaller molecular models cut from the optimized crystal lattice. Complete active space self-consistent field and multireference second-order perturbation theory calculations provide the sign and the strength of the nearest-neighbor as well as next-nearest-neighbor interactions along all three crystallographic directions. We are able to explain also intuitively the mechanism for antiferromagnetic spin coupling in terms of the superexchange pathways, discussing the role of the main four types of contributions to superexchange. Moreover, we clarify the influence of the transition metal ion on the strength of the exchange interaction and on the critical temperature for long-range ferrimagnetic ordering.
Coumarin-based dyes have been successfully used in dye-sensitized solar cells, leading to photovo... more Coumarin-based dyes have been successfully used in dye-sensitized solar cells, leading to photovoltaic conversion efficiencies of up to about 8%. Given the need to better understand the behavior of the dye adsorbed on the TiO 2 nanoparticle, we report results of density functional theory (DFT) and time-dependent DFT (TD-DFT) studies of several coumarin-based dyes, as well as complex systems consisting of the dye bound to a TiO 2 cluster. We provide the electronic structure and simulated UV-Vis spectra of the dyes alone and adsorbed to the cluster and discuss the matching with the solar spectrum. We display the energy level diagrams and the electron density of the key molecular orbitals and analyze the electron transfer from the dye to the oxide. Finally, we compare our theoretical results with the experimental data available and discuss the key issues that influence the device performance.
We report results of density functional theory (DFT) calculations of a metal-free dye, 5-(4-sulfo... more We report results of density functional theory (DFT) calculations of a metal-free dye, 5-(4-sulfophenylazo)salicylic acid disodium salt, known as Mordant Yellow 10 (MY-10), used as sensitizer for TiO 2 dye-sensitized solar cells (DSSCs). Given the need to better understand the behavior of the dyes adsorbed on the TiO 2 nanoparticle, we studied various single and double deprotonated forms of the dye bound to a TiO 2 cluster, taking advantage of the presence of the carboxyl, hydroxyl, and sulfonic groups as possible anchors. We discuss various binding configurations to the TiO 2 substrate and the charge transfer from the pigment to the oxide by means of DFT calculations. In agreement with other reports, we find that the carboxyl group tends to bind in bidentate bridging configurations. The salicylate uses both the carboxyl and hydroxyl substituent groups for either a tridentate binding to adjacent Ti(IV) ions or a bidentate Ti-O binding together with an O-H-O binding, due to the rotation of the carboxyl group out of the plane of the dye. The sulfonic group prefers a tridentate binding. We analyze the propensity for electron transfer of the various dyes and find that for MY-10, as a function of the anchor group, the DSSC performance decreases in the order hydroxyl + carboxyl > carboxyl > sulfonate. An efficient solar cell sensitizer should demonstrate (i) strong adsorption to the semiconductor surface through anchoring groups, (ii) intense absorption in the visible part of the spectrum, (iii) proper energy level alignment of the excited state of the dye and the conduction band edge of the semiconductor, as well as the redox level of the electrolyte and the ground state of the dye, (iv) fast charge transfer from the dye to the substrate, with low loss of photoelectrons, and (v) electrochemical and thermal stability [1, 4, 12].
We report the results of broken symmetry density functional theory (BS-DFT) calculations providin... more We report the results of broken symmetry density functional theory (BS-DFT) calculations providing the exchange coupling constants for two quasi-one-dimensional manganese-porphyrin compounds, [MnOEP] [HCBD] and [MnTtBuPP][HCBD] (OEP = octaethylporphyrinato, TtBuPP = meso-tetrakis-(4 0 -tert-butylphenyl)porphinato, HCBD = hexa-cyanobutadiene). The different magnetic behaviour of these materials is due mainly to the distinct binding modes of the cyanocarbon unit. We compare and contrast the results of BS-DFT calculations for magnetic dinuclears, which properly model the actual molecular magnets, to determine the geometry dependence of the exchange interaction. The exchange coupling constants resulting from BS-DFT calculations vary strongly with the functional used, hybrid functionals such as B3LYP leading to results that better correlate with the constants determined experimentally. Structure-properties correlations reveal the determinant role of the Mn-(N:C) TCNE bond angle on the ferrimagnetic coupling between the S 1 = 2 spin located on the Mn III -porphyrin donor and the S 2 = 1/2 spin positioned on the cyanocarbon acceptor. Based on a phenomenological model providing the geometry dependence of the exchange coupling constants, we fitted the results of the DFT calculations and obtained parameters describing the ferromagnetic and the antiferromagnetic parts of the superexchange interaction. The large differences between the magnetic properties of the two Mn-HCBD systems are explained based on the correlation between the exchange coupling constant and the overlap between the Mn(III) d and the HCBD p* orbitals and on the torsion angle of the butadiene backbone, which affects dramatically the nature of the p* orbital in the case of the nonplanar HCBD.
We report results of density functional theory (DFT) calculations of a metal-free dye, 5-(4-sulfo... more We report results of density functional theory (DFT) calculations of a metal-free dye, 5-(4-sulfophenylazo)salicylic acid disodium salt, known as Mordant Yellow 10 (MY-10), used as sensitizer for TiO 2 dye-sensitized solar cells (DSSCs). Given the need to better understand the behavior of the dyes adsorbed on the TiO 2 nanoparticle, we studied various single and double deprotonated forms of the dye bound to a TiO 2 cluster, taking advantage of the presence of the carboxyl, hydroxyl, and sulfonic groups as possible anchors. We discuss various binding configurations to the TiO 2 substrate and the charge transfer from the pigment to the oxide by means of DFT calculations. In agreement with other reports, we find that the carboxyl group tends to bind in bidentate bridging configurations. The salicylate uses both the carboxyl and hydroxyl substituent groups for either a tridentate binding to adjacent Ti(IV) ions or a bidentate Ti-O binding together with an O-H-O binding, due to the rotation of the carboxyl group out of the plane of the dye. The sulfonic group prefers a tridentate binding. We analyze the propensity for electron transfer of the various dyes and find that for MY-10, as a function of the anchor group, the DSSC performance decreases in the order hydroxyl + carboxyl > carboxyl > sulfonate. An efficient solar cell sensitizer should demonstrate (i) strong adsorption to the semiconductor surface through anchoring groups, (ii) intense absorption in the visible part of the spectrum, (iii) proper energy level alignment of the excited state of the dye and the conduction band edge of the semiconductor, as well as the redox level of the electrolyte and the ground state of the dye, (iv) fast charge transfer from the dye to the substrate, with low loss of photoelectrons, and (v) electrochemical and thermal stability [1, 4, 12].
We report quantum chemical calculations to address yet unresolved and puzzling questions regardin... more We report quantum chemical calculations to address yet unresolved and puzzling questions regarding the structural and magnetic disorder of V[TCNE] x (TCNE = tetracyanoethylene, x $ 2), the first room-temperature molecule-based magnet. Starting from an ideal lattice model, containing TCNE ligands either tetra-or bi-connected to vanadium(II) ions, we identify the key sources of structural disorder, explaining the amorphousness and non-stoichiometric nature of V [TCNE] x . The proposed model is prone to static disorder in terms of the bulk distribution of the tetra-connected TCNE species and to dynamic effects due to the relative rotational freedom of the bi-connected TCNE moieties. Density functional theory (DFT) calculations of the model system with rotated TCNE molecules show a rough energy landscape, consistent with the presence of magnetic irreversibilities in the system. The broken symmetry DFT approach evidences ferrimagnetic spin orientation for all TCNE configurations, ruling out the spin glass model. Multiconfigurational calculations with additional spin-orbit interaction allow for the account of the single-ion-anisotropy of the V(II) ions in different environments. We determine a small uniform zero-field-splitting (D c = À0.03 K) of the bulk as well as a sizeable random anisotropy (D r = 0.56 K) due to TCNE vacancies. We clarify the interplay of ferrimagnetism and random magnetic anisotropy in this system, which favours correlated sperimagnetic and not spin glass behaviour, in agreement with puzzling experimental data. Our approach goes beyond the material of interest here, as it can be applied to other disordered molecular magnets by correlating the sources of disorder with their effects on the magnetic properties.
ABSTRACT We report quantum chemical calculations providing the exchange coupling constants of the... more ABSTRACT We report quantum chemical calculations providing the exchange coupling constants of the V[TCNE]2 model system, describing the amorphous room temperature molecular magnet V[TCNE] x (TCNE = tetracyanoethylene, x ~ 2). The geometry is optimized for the ideal lattice using density functional theory (DFT) calculations with periodic boundary conditions. Broken-symmetry DFT calculations indicate antiparallel spin alignment resulting in ferrimagnetic ordering, but heavily overestimate the value of the exchange coupling. Better estimates of the exchange coupling parameters between the V(II) ion and the [TCNE]− anionic radical are obtained by means of multiconfigurational calculations performed on smaller molecular models cut from the optimized crystal lattice. Complete active space self-consistent field and multireference second-order perturbation theory calculations provide the sign and the strength of the nearest-neighbor as well as next-nearest-neighbor interactions along all three crystallographic directions. We are able to explain also intuitively the mechanism for antiferromagnetic spin coupling in terms of the superexchange pathways, discussing the role of the main four types of contributions to superexchange. Moreover, we clarify the influence of the transition metal ion on the strength of the exchange interaction and on the critical temperature for long-range ferrimagnetic ordering.
Coumarin-based dyes have been successfully used in dye-sensitized solar cells, leading to photovo... more Coumarin-based dyes have been successfully used in dye-sensitized solar cells, leading to photovoltaic conversion efficiencies of up to about 8%. Given the need to better understand the behavior of the dye adsorbed on the TiO 2 nanoparticle, we report results of density functional theory (DFT) and time-dependent DFT (TD-DFT) studies of several coumarin-based dyes, as well as complex systems consisting of the dye bound to a TiO 2 cluster. We provide the electronic structure and simulated UV-Vis spectra of the dyes alone and adsorbed to the cluster and discuss the matching with the solar spectrum. We display the energy level diagrams and the electron density of the key molecular orbitals and analyze the electron transfer from the dye to the oxide. Finally, we compare our theoretical results with the experimental data available and discuss the key issues that influence the device performance.
We report density functional theory (DFT) calculations of three newly synthesized oligomethine cy... more We report density functional theory (DFT) calculations of three newly synthesized oligomethine cyanine-based dyes as potential TiO2-sensitizers in dye-sensitized solar cells. The three dyes have π-symmetry and the same acceptor side, terminating in the carboxylic anchor, but they differ through the π-bridge and the donor groups. We perform DFT and time-dependent DFT studies and present the electronic structure and optical properties of the dyes alone as well as adsorbed to the TiO2 nanocluster, to provide some predictions on the photovoltaic performance of the system. We analyze theoretically the factors that can influence the short circuit current and the open circuit voltage of the dye-sensitized solar cells. We examine the matching of the absorption spectra of the dye and dye-nanocluster system with the solar irradiation spectrum. We display the energy level diagrams and discuss the alignment between the excited state of the dyes and the conduction band edge of the oxide as well ...
Http Dx Doi Org 10 1080 00268976 2013 777811, 2013
h i g h l i g h t s " The silver nanoparticles are obtained using different types of synthetic hu... more h i g h l i g h t s " The silver nanoparticles are obtained using different types of synthetic humic substances. " Synthetic humic substances act as a reducing and stabilizing agents. " The advantage of synthetic humic substances consists in their reproduced properties. " The experimental conditions were optimized to attain better yield, controlled size and high stability. " Obtained silver nanoparticles showed good antimicrobial activity.
We report results of density functional theory (DFT) calculations on some colorless aromatic syst... more We report results of density functional theory (DFT) calculations on some colorless aromatic systems adsorbed on a TiO 2 nanocluster, in order to explain experimental results regarding the photocatalytic degradation of these pollutants under visible light irradiation. Based on our modeling, we are able to clarify why transparent pollutants can degrade under visible light in the presence of a catalyst that absorbs only in the UV, to explain experimental data regarding differences in the efficiency of the degradation process, and to state the key requirements for effective water-cleaning. For that purpose, we analyze the absorption spectrum of the free and adsorbed molecules, the binding configurations, the matching of the energy levels with the oxide catalyst and the likelihood of the charge-transfer to the substrate. The comparison between several colorless aniline and phenolic systems allows a correlation between the chemical structure and the degradation rate of these pollutants.
We report quantum chemical calculations to address yet unresolved and puzzling questions regardin... more We report quantum chemical calculations to address yet unresolved and puzzling questions regarding the structural and magnetic disorder of V[TCNE] x (TCNE = tetracyanoethylene, x $ 2), the first room-temperature molecule-based magnet. Starting from an ideal lattice model, containing TCNE ligands either tetra-or bi-connected to vanadium(II) ions, we identify the key sources of structural disorder, explaining the amorphousness and non-stoichiometric nature of V [TCNE] x . The proposed model is prone to static disorder in terms of the bulk distribution of the tetra-connected TCNE species and to dynamic effects due to the relative rotational freedom of the bi-connected TCNE moieties. Density functional theory (DFT) calculations of the model system with rotated TCNE molecules show a rough energy landscape, consistent with the presence of magnetic irreversibilities in the system. The broken symmetry DFT approach evidences ferrimagnetic spin orientation for all TCNE configurations, ruling out the spin glass model. Multiconfigurational calculations with additional spin-orbit interaction allow for the account of the single-ion-anisotropy of the V(II) ions in different environments. We determine a small uniform zero-field-splitting (D c = À0.03 K) of the bulk as well as a sizeable random anisotropy (D r = 0.56 K) due to TCNE vacancies. We clarify the interplay of ferrimagnetism and random magnetic anisotropy in this system, which favours correlated sperimagnetic and not spin glass behaviour, in agreement with puzzling experimental data. Our approach goes beyond the material of interest here, as it can be applied to other disordered molecular magnets by correlating the sources of disorder with their effects on the magnetic properties.
Coumarin-based dyes have been successfully used in dye-sensitized solar cells, leading to photovo... more Coumarin-based dyes have been successfully used in dye-sensitized solar cells, leading to photovoltaic conversion efficiencies of up to about 8%. Given the need to better understand the behavior of the dye adsorbed on the TiO 2 nanoparticle, we report results of density functional theory (DFT) and time-dependent DFT (TD-DFT) studies of several coumarin-based dyes, as well as complex systems consisting of the dye bound to a TiO 2 cluster. We provide the electronic structure and simulated UV-Vis spectra of the dyes alone and adsorbed to the cluster and discuss the matching with the solar spectrum. We display the energy level diagrams and the electron density of the key molecular orbitals and analyze the electron transfer from the dye to the oxide. Finally, we compare our theoretical results with the experimental data available and discuss the key issues that influence the device performance.
ABSTRACT We report quantum chemical calculations providing the exchange coupling constants of the... more ABSTRACT We report quantum chemical calculations providing the exchange coupling constants of the V[TCNE]2 model system, describing the amorphous room temperature molecular magnet V[TCNE] x (TCNE = tetracyanoethylene, x ~ 2). The geometry is optimized for the ideal lattice using density functional theory (DFT) calculations with periodic boundary conditions. Broken-symmetry DFT calculations indicate antiparallel spin alignment resulting in ferrimagnetic ordering, but heavily overestimate the value of the exchange coupling. Better estimates of the exchange coupling parameters between the V(II) ion and the [TCNE]− anionic radical are obtained by means of multiconfigurational calculations performed on smaller molecular models cut from the optimized crystal lattice. Complete active space self-consistent field and multireference second-order perturbation theory calculations provide the sign and the strength of the nearest-neighbor as well as next-nearest-neighbor interactions along all three crystallographic directions. We are able to explain also intuitively the mechanism for antiferromagnetic spin coupling in terms of the superexchange pathways, discussing the role of the main four types of contributions to superexchange. Moreover, we clarify the influence of the transition metal ion on the strength of the exchange interaction and on the critical temperature for long-range ferrimagnetic ordering.
Coumarin-based dyes have been successfully used in dye-sensitized solar cells, leading to photovo... more Coumarin-based dyes have been successfully used in dye-sensitized solar cells, leading to photovoltaic conversion efficiencies of up to about 8%. Given the need to better understand the behavior of the dye adsorbed on the TiO 2 nanoparticle, we report results of density functional theory (DFT) and time-dependent DFT (TD-DFT) studies of several coumarin-based dyes, as well as complex systems consisting of the dye bound to a TiO 2 cluster. We provide the electronic structure and simulated UV-Vis spectra of the dyes alone and adsorbed to the cluster and discuss the matching with the solar spectrum. We display the energy level diagrams and the electron density of the key molecular orbitals and analyze the electron transfer from the dye to the oxide. Finally, we compare our theoretical results with the experimental data available and discuss the key issues that influence the device performance.
We report results of density functional theory (DFT) calculations of a metal-free dye, 5-(4-sulfo... more We report results of density functional theory (DFT) calculations of a metal-free dye, 5-(4-sulfophenylazo)salicylic acid disodium salt, known as Mordant Yellow 10 (MY-10), used as sensitizer for TiO 2 dye-sensitized solar cells (DSSCs). Given the need to better understand the behavior of the dyes adsorbed on the TiO 2 nanoparticle, we studied various single and double deprotonated forms of the dye bound to a TiO 2 cluster, taking advantage of the presence of the carboxyl, hydroxyl, and sulfonic groups as possible anchors. We discuss various binding configurations to the TiO 2 substrate and the charge transfer from the pigment to the oxide by means of DFT calculations. In agreement with other reports, we find that the carboxyl group tends to bind in bidentate bridging configurations. The salicylate uses both the carboxyl and hydroxyl substituent groups for either a tridentate binding to adjacent Ti(IV) ions or a bidentate Ti-O binding together with an O-H-O binding, due to the rotation of the carboxyl group out of the plane of the dye. The sulfonic group prefers a tridentate binding. We analyze the propensity for electron transfer of the various dyes and find that for MY-10, as a function of the anchor group, the DSSC performance decreases in the order hydroxyl + carboxyl > carboxyl > sulfonate. An efficient solar cell sensitizer should demonstrate (i) strong adsorption to the semiconductor surface through anchoring groups, (ii) intense absorption in the visible part of the spectrum, (iii) proper energy level alignment of the excited state of the dye and the conduction band edge of the semiconductor, as well as the redox level of the electrolyte and the ground state of the dye, (iv) fast charge transfer from the dye to the substrate, with low loss of photoelectrons, and (v) electrochemical and thermal stability [1, 4, 12].
We report the results of broken symmetry density functional theory (BS-DFT) calculations providin... more We report the results of broken symmetry density functional theory (BS-DFT) calculations providing the exchange coupling constants for two quasi-one-dimensional manganese-porphyrin compounds, [MnOEP] [HCBD] and [MnTtBuPP][HCBD] (OEP = octaethylporphyrinato, TtBuPP = meso-tetrakis-(4 0 -tert-butylphenyl)porphinato, HCBD = hexa-cyanobutadiene). The different magnetic behaviour of these materials is due mainly to the distinct binding modes of the cyanocarbon unit. We compare and contrast the results of BS-DFT calculations for magnetic dinuclears, which properly model the actual molecular magnets, to determine the geometry dependence of the exchange interaction. The exchange coupling constants resulting from BS-DFT calculations vary strongly with the functional used, hybrid functionals such as B3LYP leading to results that better correlate with the constants determined experimentally. Structure-properties correlations reveal the determinant role of the Mn-(N:C) TCNE bond angle on the ferrimagnetic coupling between the S 1 = 2 spin located on the Mn III -porphyrin donor and the S 2 = 1/2 spin positioned on the cyanocarbon acceptor. Based on a phenomenological model providing the geometry dependence of the exchange coupling constants, we fitted the results of the DFT calculations and obtained parameters describing the ferromagnetic and the antiferromagnetic parts of the superexchange interaction. The large differences between the magnetic properties of the two Mn-HCBD systems are explained based on the correlation between the exchange coupling constant and the overlap between the Mn(III) d and the HCBD p* orbitals and on the torsion angle of the butadiene backbone, which affects dramatically the nature of the p* orbital in the case of the nonplanar HCBD.
We report results of density functional theory (DFT) calculations of a metal-free dye, 5-(4-sulfo... more We report results of density functional theory (DFT) calculations of a metal-free dye, 5-(4-sulfophenylazo)salicylic acid disodium salt, known as Mordant Yellow 10 (MY-10), used as sensitizer for TiO 2 dye-sensitized solar cells (DSSCs). Given the need to better understand the behavior of the dyes adsorbed on the TiO 2 nanoparticle, we studied various single and double deprotonated forms of the dye bound to a TiO 2 cluster, taking advantage of the presence of the carboxyl, hydroxyl, and sulfonic groups as possible anchors. We discuss various binding configurations to the TiO 2 substrate and the charge transfer from the pigment to the oxide by means of DFT calculations. In agreement with other reports, we find that the carboxyl group tends to bind in bidentate bridging configurations. The salicylate uses both the carboxyl and hydroxyl substituent groups for either a tridentate binding to adjacent Ti(IV) ions or a bidentate Ti-O binding together with an O-H-O binding, due to the rotation of the carboxyl group out of the plane of the dye. The sulfonic group prefers a tridentate binding. We analyze the propensity for electron transfer of the various dyes and find that for MY-10, as a function of the anchor group, the DSSC performance decreases in the order hydroxyl + carboxyl > carboxyl > sulfonate. An efficient solar cell sensitizer should demonstrate (i) strong adsorption to the semiconductor surface through anchoring groups, (ii) intense absorption in the visible part of the spectrum, (iii) proper energy level alignment of the excited state of the dye and the conduction band edge of the semiconductor, as well as the redox level of the electrolyte and the ground state of the dye, (iv) fast charge transfer from the dye to the substrate, with low loss of photoelectrons, and (v) electrochemical and thermal stability [1, 4, 12].
We report quantum chemical calculations to address yet unresolved and puzzling questions regardin... more We report quantum chemical calculations to address yet unresolved and puzzling questions regarding the structural and magnetic disorder of V[TCNE] x (TCNE = tetracyanoethylene, x $ 2), the first room-temperature molecule-based magnet. Starting from an ideal lattice model, containing TCNE ligands either tetra-or bi-connected to vanadium(II) ions, we identify the key sources of structural disorder, explaining the amorphousness and non-stoichiometric nature of V [TCNE] x . The proposed model is prone to static disorder in terms of the bulk distribution of the tetra-connected TCNE species and to dynamic effects due to the relative rotational freedom of the bi-connected TCNE moieties. Density functional theory (DFT) calculations of the model system with rotated TCNE molecules show a rough energy landscape, consistent with the presence of magnetic irreversibilities in the system. The broken symmetry DFT approach evidences ferrimagnetic spin orientation for all TCNE configurations, ruling out the spin glass model. Multiconfigurational calculations with additional spin-orbit interaction allow for the account of the single-ion-anisotropy of the V(II) ions in different environments. We determine a small uniform zero-field-splitting (D c = À0.03 K) of the bulk as well as a sizeable random anisotropy (D r = 0.56 K) due to TCNE vacancies. We clarify the interplay of ferrimagnetism and random magnetic anisotropy in this system, which favours correlated sperimagnetic and not spin glass behaviour, in agreement with puzzling experimental data. Our approach goes beyond the material of interest here, as it can be applied to other disordered molecular magnets by correlating the sources of disorder with their effects on the magnetic properties.
ABSTRACT We report quantum chemical calculations providing the exchange coupling constants of the... more ABSTRACT We report quantum chemical calculations providing the exchange coupling constants of the V[TCNE]2 model system, describing the amorphous room temperature molecular magnet V[TCNE] x (TCNE = tetracyanoethylene, x ~ 2). The geometry is optimized for the ideal lattice using density functional theory (DFT) calculations with periodic boundary conditions. Broken-symmetry DFT calculations indicate antiparallel spin alignment resulting in ferrimagnetic ordering, but heavily overestimate the value of the exchange coupling. Better estimates of the exchange coupling parameters between the V(II) ion and the [TCNE]− anionic radical are obtained by means of multiconfigurational calculations performed on smaller molecular models cut from the optimized crystal lattice. Complete active space self-consistent field and multireference second-order perturbation theory calculations provide the sign and the strength of the nearest-neighbor as well as next-nearest-neighbor interactions along all three crystallographic directions. We are able to explain also intuitively the mechanism for antiferromagnetic spin coupling in terms of the superexchange pathways, discussing the role of the main four types of contributions to superexchange. Moreover, we clarify the influence of the transition metal ion on the strength of the exchange interaction and on the critical temperature for long-range ferrimagnetic ordering.
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Papers by Petre Panait